The removal of particulates from a gas stream has long been a practice in a variety of industrial fields. Conventional means for filtering particulates and the like from gas streams include, but are not limited to, filter bags, filter tubes, filter panels and filter cartridges. For convenience herein, the term “filter element” will be used to refer collectively to these types of filtration means.
Selection of the type of filtration media used is typically based on the fluid stream with which the filter element comes in contact, the operating conditions of the system and the type of particulates being filtered.
The flow of fluids, whether liquid or gas, produces a pressure differential, or pressure drop, across the element. Preferably, the pressure differential is as small as possible for a given fluid flow rate in order to minimize the power required to filter the fluid.
Filter media may be broadly characterized as either depth filtration media or surface filtration media. Particles tend to penetrate somewhat and accumulate within depth filtration media. In contrast, the majority of particles collect on the surface of surface filtration media.
Many materials are known to be useful as depth filtration media, including spunbond or meltblown webs, felts and fabrics made from a variety of materials, including polyesters, polypropylenes, aramids, cellulose, glasses and fluoropolymers. Known meltblown filter media demonstrate high efficiency and low pressure drop. Meltblown filter media also have high dust capacity. However, meltblown filter media suffer from relatively low water entry pressures, which may make them unsuitable for outdoor use in some environments.
Surface filters, such as membranes, have gained popularity in certain applications, particularly outdoor environments or those in which the fluid to be filtered contains liquid aerosols or harsh chemicals. In other applications, membrane filter media is useful because it has a more constant filtration efficiency than that of depth filtration media. Membranes have stable filtration efficiency because, unlike depth filtration media, a membrane filter's efficiency is not dependent upon the buildup of a cake of dust particles.
Polytetrafluoroethylene (PTFE) has demonstrated utility in many areas such as harsh chemical environments, which normally degrade many conventional metals and polymeric materials. A significant development in the area of particle filtration was achieved when expanded PTFE (ePTFE) membrane filtration media were incorporated as surface laminates on conventional filter elements. Examples of such filtration media are taught in U.S. Pat. Nos. 4,878,930, and 5,207,812, which are directed to filter cartridges for removing particles of dust from a stream of moving gas or air. Membranes constructed of ePTFE are advantageously hydrophobic.
In known filter systems where thermoplastic and ePTFE layers are bonded together to form filter elements, the bonded regions between separate layers may significantly restrict the fluid flow through the filter elements resulting in lower filter system performance. Such may be the case where conventional lamination techniques of bonding multiple layers by the application of heat and pressure across the entire surface of the filter element. In such filters, the uncontrolled intermingling of materials from the thermoplastic layer creates regions of restricted flow. Furthermore, the application of heat and pressure across the entire surface may cause stress and compression in large regions of the ePTFE layer, reducing filtration efficiency and lowering water entry pressure. In some outdoor environments, the loss of filtration efficiency and water entry pressure may lead to early failure of the filter element.